The present invention relates generally to telescopic optical systems and more particularly relates to a telescopic optical system designed to be pointed at an object of interest which is only slightly angularly displaced in space from the sun. When the object of interest is near the sun, such as within 1.degree. of the sun's limb, and even when the sun is out of the field of view of the optical system, solar radiation diffracted into the optical system can be substantially more powerful than radiation forming the image of interest. In the problem which resulted in the present invention, a reduction of 5,000 times the intensity of diffracted solar energy was required.
A standard solution to this problem in the prior art has been to use a re-imaging system having a field stop and a Lyot stop. In this solution a separate optical system is utilized to re-image the image formed by the main telescopic optical system. A Lyot stop is utilized to block out the edges of the entrance aperture and to only allow radiation entering the aperture away from its edges to form the new image. In this manner radiation diffracted at the edges of the entrance aperture is blocked out along with the edges of the entrance aperture. The main disadvantage of this approach is that it inherently adds a significant level of complexity and weight to the original telescopic system.
The concept of apodization of an entrance aperture is known. Section 8.49 of the textbook Modern Optics, by Earle B. Brown, published by Reinhold Publishing Corp., 1965, defines apodization.
Apodization has been tried in the prior art to change the shape of the central maximum and also to alter the amount of radiation in the diffraction field immediately surrounding the central maximum. A major disadvantage of such classical apodization techniques is that relatively large changes are required in the transmission properties of the entrance aperture to affect the desired change in radiation in the central maximum and the diffraction field immediately surrounding the central maximum. The substantial changes in the entrance aperture resulted, in many instances, in a substantial reduction in the radiation actually utilized by the optical system. Also, there are major engineering problems in making the transmission filters required by those systems.
When the parameters and requirements of the present system were defined, apodization of the entrance aperture was considered by optical experts as a possible solution. It was concluded that apodization would not work as it was thought that too much radiation would be eliminated.
The present invention was recognized by the inventors as a practical solution to the problem as they realized that apodization of the entrance aperture to change the intensity of radiation in the outer diffraction rings far from the central maximum involves a different type of problem than apodization to change the intensity of radiation in the diffraction field immediately surrounding the central maximum.